1) Field of the invention
The present invention relates to a method of controlling a continuous carburization furnace, and in more particular to a method of controlling the continuous carburization furnace when members to be carburized are changed into members of a different kind having a different carburizing condition.
2) Description of the Related Art
Conventionally, for structural components, a case hardening process which is referred to as carburization has been executed wherein the case layer is hardened but the core remains tenacious. As a result, the process provides an anti-shock characteristic in that an inside tenaciousness reinforces the brittleness of the hardened case with a resistivity against wear.
FIG. 1 is a schematic structural view of the conventional pusher type continuous carburization furnace for carrying out the carburization process using a gas suitable for carburization. The continuous carburization furnace is normally divided into zones for partitioning a temperature and a furnace atmosphere by a partitioning arch W. The furnace includes a temperature rising zone, a carburization zone, a diffusion zone and a quench hardening zone. The member to be carburizably processed is placed on a tray TR by a jig etc., and inserted sequentially into the furnace from an inlet by means of the pusher Pl. For example in this furnace, thirteen trays TR are provided from the temperature rising zone to the diffusion zone in the furnace and pushed by the pusher Pl to move at a predetermined distance in the furnace every time the predetermined time lapses and to stop at the thirteen carburization positions during the foregoing predetermined interval, thereby performing the carburization. The trays TR pushed out from the diffusion zone are inserted into the quench hardening zone by the pusher P2 and are moved in the quench hardening zone by the pusher P3 to reach an outlet.
In the carburization furnace constituted as described above, a turn ON and OFF control of a heater H by a heater control or an adjustment of the inclusion amount of butane gas to the gas from a gas inlet G by means of an atmospheric gas control is executed so that the temperature and carbon potential detected by a detector S are obtained to satisfy the characteristics as shown in FIGS. 2 and 3, the carbon potential (a carburized depth) of the member to be carburized is thus adjusted. For example, the carburized depth (the carbon potential) of automobile parts are 1.5 mm or more for cam shafts and piston pins, and in the order of 1.0 to 1.5 mm for ring gears, bearing rollers and transmission gears, and further 0.5 mm or less for push rods and shackle bolts.
However, in the conventional continuous carburization furnace, when the conditions of respective zones vary with each change of the members to be processed, a number of vacant trays must be sent until the time when the immediately previous processed members are discharged and the time when the previous conditions of the respective zones become new conditions for the next members to be carburizably processed, a problem thus arises because such a process degrades the productivity of the carburization furnace. In this case, if the members to be processed with the other different conditions are inserted continuously into the furnace without sending the vacant trays, a disadvantage occurs in that the carburized depth of the member fluctuates, i.e., increased more than or lowered less than the reference carburized depth of the member.
Further, in the control of the conventional continuous carburization furnace, since the operational conditions of the respective zones are controlled to set the temperature and the carburization potential at a constant value and not controlled based on the temperature or atmospheric conditional information received by each tray, then changes of the operational conditions cause fluctuations of the carburized depths as it is.
To prevent the fluctuation of the carburized depths when placed on every tray, in the conventional method, it is determined whether or not the carburized depth at every tray is satisfactory by using the test members, i.e., the carburized depth of an object to be determined that is placed on each tray is estimated by measuring the carburized depths of the test members that are placed on the trays and carburized under the same conditions. Therefore, this conventional method requires excess test members and a lot of time for measuring the carburizing degree of the test members, and has often resulted in defective products.